1. Field of the Invention
The present invention relates to voltage supply circuits and, particularly, to a voltage supply circuit for supplying a voltage to a sensor such as a capacitor microphone, a microphone unit using the same, and a method of adjusting the sensitivity of the microphone unit.
2. Description of Related Art
Technology that uses a microphone called a capacitor microphone for audio communication in mobile terminals such as cell phones is widely used. The capacitor microphone is a microphone which is formed of a capacitor whose one electrode is a diaphragm and which perceives the vibration of a sound or the like as a change in capacitance and converts it into an electric signal.
“PA Audio-communication System” (Kougakutosho Ltd., 1996) describes a normal capacitor microphone system. FIG. 20 shows a circuit of a capacitor microphone unit 100 that uses the capacitor microphone. As shown in FIG. 20, the conventional capacitor microphone unit includes a capacitor microphone 101, a junction field-effect transistor (JFET) 102, a capacitor 103, resistors 104 and 105, and DC power supplies 106 and 108.
The capacitor microphone 101 is a vibration sensor that receives a sound pressure such as a sound and generates an output signal. One electrode of the capacitor microphone 101 is connected to the DC power supply 108 through the resistor 104, and the other electrode is grounded. The capacitor microphone 101 receives a specific bias voltage from the DC power supply 108. The output terminal of the capacitor microphone 101 is connected to the gate of the JFET 102. The JFET 102 is an amplifier that amplifies an output signal from the capacitor microphone 101 and generates an amplified signal. The amplified signal generated in the JFET 102 is output from the capacitor microphone unit 100 through an output terminal 107.
Though the capacitor microphone unit 100 shown in FIG. 20 uses two DC power supplies 106 and 108, it is feasible to boost the voltage supplied from the DC power supply 106 to generate a bias voltage to be supplied to the capacitor microphone 101.
In such a capacitor microphone unit, manufacturing variation can occur in the manufacture of a capacitor microphone and JFET. The manufacturing variation leads to variation in distance between capacitor electrodes and variation in amplification efficiency of JFET, causing capacitor microphone units to have different sensitivities.
Therefore, there has been a need for a capacitor microphone unit that can operate with an appropriate sensitivity even if manufacturing variation occurs in a sensor device such as a capacitor microphone.
Further, conventional techniques change the sensitivity of a capacitor microphone unit by preparing two capacitor microphone units with different sensitivity settings and switching the capacitor microphone units themselves. In this configuration, it is necessary to prepare the same number of capacitor microphone units as the number of sensitivity levels to be switched. Therefore, there has been a need for a capacitor microphone unit that allows selection between a plurality of sensitivity levels in one unit.